Physics
General Relativity
Quick fact
GPS satellites must account for general relativity, or they'd be off by kilometers each day.
Why this is interesting
Have you ever wondered why gravity feels like an invisible force? What if gravity isn't a force at all, but a warping of space and time itself?
Read the full explanation
Understanding General Relativity
Imagine space and time as a stretchy rubber sheet. Place a heavy ball (like the Sun) on it, and the sheet curves around it. Now roll a marble (like Earth) near the ball—it follows the curved path, not because the ball pulls it, but because the sheet's shape guides it. In general relativity, mass and energy curve the four-dimensional fabric of spacetime. Objects, including light, move along these curves, which we experience as gravity. This explains why planets orbit stars and why light bends around massive galaxies.
A deeper explanation
General relativity, formulated by Albert Einstein in 1915, is based on the idea that gravity arises from spacetime curvature. The theory's core is the Einstein field equations, which link the distribution of matter and energy to the curvature of spacetime. This curvature dictates the paths (geodesics) of free-falling objects. The equivalence principle—that gravitational and inertial effects are indistinguishable—is a key foundation. General relativity predicts phenomena such as the precession of Mercury's orbit, gravitational time dilation (clocks run slower in stronger gravity), and the existence of black holes where curvature becomes infinite. It also predicted gravitational waves, ripples in spacetime confirmed directly in 2015. The theory is essential for understanding the large-scale structure of the universe, from stellar evolution to cosmology, and is verified by experiments like gravitational lensing and the bending of starlight during solar eclipses.